We examined whether submaximal treadmill exercise during acute isovolumic anemia altered the distribution of myocardial blood flow and thus caused subendocardial ischemia in unsedated dogs. Myocardial blood flow (determined by the microsphere method) and left ventricular function (indicated by pressures, volumes, and contractile indices) were measured in five dogs at rest and during exercise before and after equal volume exchange of blood and 6% dextran 70, which lowered hematocrit from 36 +/- 4 to 18 +/- 2% (SD). Total myocardial blood flow increased from 175 +/- 65 to 296 +/- 151 ml . min-1 . 100 g-1 (69%) during exercise before anemia and from 329 +/- 61 to 599 +/- 126 (82%) during exercise after anemia. The distribution of blood flow to the left and right ventricles and interventricular septum as well as the subendothelial-to-subepicardial blood flow ratio in these areas did not change during exercise either before or after anemia. Left ventricular function was not impaired during exercise after anemia. We conclude that subendocardial ischemia does not occur when dogs exercise during acute isovolumic anemia.